510 POWER OP FLIGHT POSSESSED BY BIRDS. 
during the down-stroke; otherwise the effect of the former 
would neutralise that of the latter. This is partly accom¬ 
plished by the great velocity of the down-stroke compared 
with the up-stroke, which causes the resistance of the air to be 
much greater against the former than against the latter. 1 But 
it is by the alteration in the surface of the wing, as it acts upon 
the air, that the chief difference is made in Birds; the arrange¬ 
ment of their great feathers being such, that they strike the air 
with their flat sides, but present only their edges in rising. 
What is called “ feathering the oar ” in rowing, is a similar 
operation, performed with the same intention, and deriving 
its name from this resemblance. 
67 2. The degree in which the wings act in raising the body 
or in propelling it through the air, varies considerably in 
different species, according to the way in which they are set. 
Thus in Birds of Prey, which require a rapid horizontal 
motion, the surface of the wings is very oblique, so that they 
strike backwards as well as downwards, and thus impel the 
body forwards whilst sustaining it in the air. Such birds find 
a difficulty in rising perpendicularly ; and can in fact only do 
so by flying against the wind, which then acts upon the 
inclined surface of the wings just as it does upon that of a 
kite. On the other hand, the Lark, Quail, and such other 
birds as rise to great heights in a direction nearly vertical, 
have the wings so disposed as to strike almost directly down¬ 
wards. It has been estimated that a Swallow, when simply 
sustaining itself in the air, is obliged to use as much force to 
prevent its fall, as would raise its own weight to a height of 
about twenty-six feet in a second. Hence, we may form some 
idea of the enormous expenditure of force which must take 
place, when the body is not only supported, but raised and 
propelled through the air. The Eider-duck is said to fly 
90 miles in an hour, and the Hawk 150. The Swallow 
and Swift pass nearly the whole of the long summer days 
upon the wing, in search of food for themselves and their 
1 This resistance varies as the square of the velocity of the stroke. 
Hence, if the down-stroke be made three times as fast as the up-stroke, 
the resistance it experiences will be nine times as great. But as this 
only operates during one-third of the time, it is in effect equal to three 
times that which operates against the up-stroke, and which would tend 
to lower the Bird in the air. 
